One of the most important problems in medicine and biology is measuring the flow of blood to organs. Adequate blood flow is essential to the health of organs as the flow of blood carries required nutrients to the organs and waste products away from the organs. Perfusion refers to the passage of blood through a vascular bed, usually capillaries, although the definition can also include both arterioles and venules. The flow of blood proceeds first through a branching network of arteries which spread out from the heart in much the same manner as the branches of a tree spread out from the trunk. At the end of the arterial network, the blood flows through capillaries which connect the network of arteries to the network of veins. The majority of nutrient/waste transfer occurs in the capillaries. Blockage anywhere in the vascular system will impede blood flow. When blood flow in a major artery or vein is diminished, so too is the flow of blood to all capillaries connected to that vessel. If the flow of blood ceases for a long period of time, then the quiescent blood will clot, thus eliminating the obstructed vessels as viable pathways for further blood flow.
Fall in tissue perfusion below a critical level results in tissue injury, cell death, and impaired organ function. This is a dynamic process termed ischemia-reperfusion injury which has several elements: impaired perfusion for a critical period of time, such that both leukocytes and vascular endothelium are activated, followed by further tissue injury when perfusion is reestablished. From this brief description, it is apparent that time is of the utmost importance in terms of limiting tissue injury, and precise measurements of tissue perfusion are essential for managing the condition. A wide variety of diseases are associated with impaired tissue perfusion and, when present to a major degree, the risk of full-blown ischemia-reperfusion injury of such vital organs as the heart, brain, kidneys, and gut. Conditions associated with impaired tissue perfusion include sepsis, hepatic cirrhosis, heart failure, shock of diverse etiologies, the post-operative state after major surgery, and critical illnesses of a variety of kinds. Powerful drugs are available to modify tissue perfusion, if reliable measurement technologies were available to assess the status of perfusion and the effects of an intervention. Such a strategy would help greatly in preventing or limiting the extent of renal failure, myocardial infarction, stroke, and other consequences of hypoperfusion and ischemia-reperfusion injury.
Such an approach would also have a role in the management of solid tumors. Extensive studies over the past 15 years have shown that tumor angiogenesis is an important element of the growth and development of solid tumors. Anti-cancer strategies that involve anti-angiogenesis drugs that decrease tumor perfusion are now in advanced clinical testing. A technology for assessing perfusion of tumor mass as a guide to clinical management is greatly to be desired.